The efficacy and safety of electroconvulsive therapy (ECT) for the treatment of depression has been proven. The mechanism remains unclear. Authors have focused on ECT's anticonvulsant effect as a mechanism of action. The anticonvulsant properties of ECT are evidenced by increase in seizure threshold and decreased seizure duration and by its effects on inhibitory neurotransmitters, inhibitory peptides; and by the inhibitory effect on amygdala-kindled seizures in animals. As most ECT-induced seizures end spontaneously, the process that underlies the termination of these seizures may be related to the treatment's anticonvulsant effect, and, therefore, to its mechanism of action. There are recent reports of regional cerebral blood flow (rCBF) reduction after ECT-induced seizures in delayed studied (i.e., >50 minutes after the treatment). Patients who responded to the treatment had larger reductions in cerebral blood flow than nonresponders. These reductions were most pronounced after the initial treatment and disappeared by the seventh treatment (8). None of these rCBF studies has focused on the immediate post-ictal period and all have been done utilizing the Xenon inhalation method with single- photon emission computed tomography (SPECT). The study attempts to establish a correlation of electroencephalographic and rCBF changes measured with PET, and the effect of repeated treatments on rCBF changes. The effects of balanced anesthesia on rCBF will be studied. The use of PET allows a better visualization of small brain regions.